1. Field of the Invention
The present invention relates to a flash fixing device and an image forming device. More particularly, the present invention relates to a flash fixing device which fixes a toner image, which has been transferred onto a recording medium, by illuminating flash light emitted from flash lamps, and to an image forming device equipped with the flash fixing device.
2. Description of the Related Art
An image forming device, which forms images by an electrophotographic method, transfers, onto a recording medium, a toner image formed by powder toner, and thereafter, applies thermal energy to the recording medium on which the toner image is transferred (i.e., to the powder toner on the recording medium) and fuses the powder toner, thereby fixing the toner image on the recording medium. A heat roller is usually used in order to supply the thermal energy for toner image fixing. However, high-capacity image forming devices which can form a large amount of images at high speed (e.g., an image forming device which can form images on recording medium of a surface area of about 500 A4-sized sheets per second) use a flash fixing system which illuminates flash lamps intermittently, and by illuminating the flash light which is emitted from the flash lamps when the flash lamps are lit, supplies the energy for fusing the powder toner and fixing the toner image. Because the flash fixing system can supply high energy without contacting the recording medium, it does not affect the conveying of the recording medium and is suitable for high-speed image formation.
In its main applications, document printing in high-capacity image forming devices is usually monochrome. However, even in document printing the need for color printing is gradually increasing for high-capacity image forming devices, such as, for example, when it is desired to print in color a company logo to be added as a header or a footer to a document, or the like. Formation of color images in an electrophotographic system can be realized by superposing toner images of the respective colors of C, M, Y (and K). However, accompanying this, the amount of toner which is transferred onto the recording medium (the amount of toner which is the object of fixing) increases, and the need arises to supply a large amount of energy in order to fix the toner image.
In a flash fixing system, increasing the energy which is supplied can be realized by decreasing the speed of conveying the recording medium (for example, if the conveying speed is halved, the supplied energy is doubled), or by shortening the light-emission cycle time of the flash lamps (for example, if the light-emission cycle time is halved (the light-emission frequency is doubled), the supplied energy is doubled). However, there are the problems that decreasing the conveying speed of the recording medium is not desirable, as it relates to a deterioration in the processing capacity of the image forming device, and shortening the light-emission cycle time of the flash lamps as well leads to shortening of the lifespan of the flash lamps and the rise in the temperature of the lamps also is great. Therefore, in order to increase the supplied energy without decreasing the conveying speed or shortening the light-emission cycle time, there have come to be generally employed structures in which plural flash lamps are arranged along the conveying direction of the recording medium, and by causing these flash lamps to emit light simultaneously, flash light is illuminated over a relatively broad surface area on the recording medium in the light emission of a single time.
In connection with the above, Japanese Patent Application Laid-Open (JP-A) No. 2001-142347 discloses a flash fixing device equipped with a flash power source section which supplies electric power such that, during the time when the flash lamps are emitting light, the discharge current flowing to the flash lamps is substantially even.
However, in a case in which plural flash lamps, which are arranged along the conveying direction of the recording medium, are made to emit light simultaneously, the light distribution pattern becomes a pattern in which, as shown in FIG. 25A as an example, the amount of illuminated light of the flash light is substantially uniform at the central portion in the recording medium conveying direction, but, in vicinities of the two end portions in the recording medium conveying direction, the further away from the central portion, the more the amount of illuminated light of the flash light decreases gently. Note that, although FIG. 25A illustrates a case in which there are four flash lamps, however the light distribution pattern becomes a pattern in which the amount of illuminated light of the flash light decreases in vicinities of the two end portions as described above if the number of flash lamps is plural, even if it is other than four.
Therefore, in a case in which flash light is illuminated onto a recording medium by causing plural flash lamps to emit light simultaneously, a conveying speed v of the recording medium and a light-emission frequency f of the flash lamps (f=1/light-emission period T) are adjusted such that, at the portions of the recording medium illuminated by the flash light corresponding to the low light amount portions in the light distribution pattern (the vicinities of the two end portions), the flash light corresponding to the low light amount portions is illuminated two times, as shown in FIG. 25B. Note that, at the recording medium, a length S along the recording medium conveying direction of a region where the flash light is illuminated only one time (called a “single-flash region”), and a length D along the recording medium conveying direction of a portion where the flash light is illuminated two times (called a “repeat region”), are such that D=W−v/f and S=W−D, where W is the width of the opening of the flash lamp unit (see FIG. 25A).
However, due to the amount of the toner to be fixed increasing due to the image being formed in color as described above, the problem arises that the image quality deteriorates in the aforementioned repeat regions. Namely, in the stage before the flash light is illuminated onto the recording medium, the toner on the recording medium is all in a powder state as shown by (1) in FIG. 26B. When the flash light is illuminated, energy is supplied, and fusing of the toner occurs. However, although the flash light is illuminated two times in the repeat region, the amount of illuminated light of the flash light in the illumination each time is small, and therefore, the toner surface temperature and the recording medium interface temperature at the repeat region vary as shown in FIG. 26A.
More specifically, during the illumination of the flash light the first time, the surface of the toner at the upper layer side in the repeat region fuses due to the temperature exceeding the toner fusing temperature (softening). However, the temperature of the toner of the lower layer side in the repeat region does not reach the toner fusing temperature, and therefore, fusing does not occur (see (2) in FIG. 26B). Further, during the period of time from after the illumination of flash light the first time is carried out and time corresponding to the light-emission period T elapses and the illumination of flash light the second time is carried out, energy is not supplied to the toner at the repeat region. Therefore, the temperature of the toner decreases due to heat dissipation, and the fused toner of the upper layer side begins to cohere due to surface tension (see (3) in FIG. 26B). Further, due to the temperature of the toner at the upper layer side decreasing greatly during the period of time until the illumination of the flash light is carried out a second time, the toner of the lower layer side which is in the state of a powder is pulled, due to the strong surface tension of the toner of the upper layer side, and deterioration in image quality, such as missing dots (white spots) called voids where the surface of the recording medium is uncovered, or the like, arises (see (4) of FIG. 26B).